1. Field of the Invention
The present invention relates to a switching regulator type power supply circuit and, more particularly, to a switching regulator type power supply circuit using a blocking oscillation circuit.
2. Description of the Prior Art
A switching regulator type power supply circuit is classified into various types and one of them is a type in which blocking oscillation is performed by a switching transistor and a converter transformer. One of power supply circuits of this type as shown in FIG. 1 is disclosed in Japanese Patent Laying-Open No. 2570/1984.
The power suply circuit shown in FIG. 1 comprises an input rectifying portion 1, a blocking oscillation portion 2, a converter transformer 3, a fluctuation detecting portion 4, a control portion 5, an output rectifying portion 6 and a feedback current control circuit 7 only the portion relating to the present invention is described hereinafter.
The conventional example shown in FIG. 1 performs a blocking oscillation, when a switching transistor TR4 is on, by letting a positive feedback current If flow from the side of a terminal e of a feedback winding N3 of the converter transformer 3 to the base of the transistor TR4 through a path shown in the figure. The feedback current control circuit 7 comprising a constant-current circuit CA in the current path is provided to cause the positive feedback current If to be a constant current in a steady state. A more detailed description is made in this respect. Since a Zener diode D10 and resistors R16-R18 are selected such that a transistor TR5 in the circuit 7 may be turned on by the voltage between the terminals c and e of the feedback winding N3 while the switching transistor TR4 is on and the resistor R12 is set sufficiently larger than the resistor R17, the above mentioned positive feedback current If flows mainly through the transistor TR5, and the current becomes a constant current due to If=(V.sub.Z -V.sub.BE)/R17 where V.sub.BE is the voltage between the base and the emitter of the transistor TR5 and V.sub.Z is the Zener voltage of the Zener diode D10.
The above positive feedback current If is caused to be a constant current for the following reason. Assuming that the feedback current control circuit 7 is not a constant-current circuit but a constant-impedance, the positive feedback current If in this case is proportional to the voltage between the terminals c and e of the feedback winding N3 and that voltage is proportional to an input voltage applied to an input winding N1. As a result, the more the input voltage applied to the input winding N1 is decreased, the more the current If is decreased. Meanwhile, referring to FIG. 1, when the input voltage applied to the input winding N1 is decreased or a load connected to an output winding N2 is increased, an "on period" of the switching transistor TR4 is controlled to be long, so that a collector current Ii of the TR4 is increased at that time. Therefore, in that case, the base positive feedback current If large enough to allow a flow of the collector current Ii of a big peak value can not be supplied and the switching transistor TR4 is not driven, with the result that a predetermined DC output voltage can not be provided from the output winding N2. Thus, to avoid such a defect, the positive feedback current If is made to be a constant current.
However, in a conventional circuit shown in FIG. 1, if the current value has been set to be a relatively high value so as to supply enough positive feedback current If when the input voltage applied to the input winding N1 is low or the load connected to the output winding N2 is increased, the switching transistor TR4 is in an overdrive state when the input voltage applied to the input winding N1 is high or the load connected to the output winding N2 is decreased, with the result that a stabilized control range becomes small due any increase in the power loss of the switching transistor TR4. In addition, when the input voltage applied to the input winding N1 is high, the voltage of the feedback winding is also high as mentioned above, so that power consumption in the transistor TR5 is also increased.
Meanwhile, although the positive feedback current If is always kept at a constant value while the switching transistor TR4 is on, the collector current Ii is increased linearly as time passes while the switching transistor TR4 is on and, as a result, the above current If can not be a value providing an optimum drive always during the above mentioned on period. Therefore, there is also another disadvantage that the power loss of the switching transistor TR4 is originally high without regard to the state of the input voltage applied to the input winding N1 and the load connected to the output winding N2.